1. Field of the Invention
The present invention relates to an input change-over type amplifier and a frequency change-over type oscillator formed by using the input change-over type amplifier. In particular, this invention relates to an input change-over type amplifier capable of switching by a switch to amplify and output two signals having different frequencies. The present invention also relates to a frequency change-over type oscillator formed by using such an amplifier.
2. Description of the Related Art
FIG. 6 is an explanatory view schematically indicating a frequency change-over type oscillator 11 made according to a prior art. In fact, a basic principle of the frequency change-over type oscillator 11 shown in FIG. 6 has been disclosed in Japanese Unexamined Patent Publication No. 10-126152.
Referring to FIG. 6, the conventional frequency change-over type oscillator 11 has an input change-over type amplifier 10, a first signal source 51 and a second signal source 52. A transistor 2 of the input change-over type amplifier 10 is an amplifying element. In detail, the base of the transistor 2 serves as an input terminal, its emitter serves as a ground terminal, and its collector serves as an output terminal. In practice, the emitter of the transistor 2 is connected to a grounding circuit 40 which is connected with GND via a ground capacitor 400. The base of the transistor 2 is connected through a connection point 24 to both the first signal source 51 and the second signal source 52. From the first and the second signal sources 51 and 52, either a first high frequency signal s1 or a second high frequency signal s2 is output, so that the signal s1 or the signal s2 is allowed to pass through the connection point 24 so as to be fed into the base of the transistor 2. The ground capacitor 400 of the grounding circuit 40 is so designed that it has a sufficiently small impedance with respect to a high frequency current. Therefore, when a high frequency signal s1 or a high frequency signal s2 is fed to the base of the transistor 2, the transistor 2 becomes grounded through its emitter.
However, with the frequency change-over type oscillator 11 shown in FIG. 6, since a portion of the signal s1 output from the first signal source 51 is transmitted into the second signal source 52, the level of the signal to be input into the base of the transistor 2 is reduced. The same is true with the signal source s2, hence the level of a signal to be input into the base of the transistor 2 is reduced undesirably. Namely, since the two signal sources are both connected with the connection point 24, a signal output from one signal source flows into the other signal source, the level of a signal to be input into the base of the transistor 2 is reduced undesirably. As a result, when the level of a signal to be input into the base of the transistor 2 is lowered, there is a problem that the level of a signal to be output from the collector of the transistor 2 also deteriorates. Moreover, when the level of a signal output from the collector of the transistor 2 becomes low, there is a problem that it will be difficult to ensure a stability with respect to an external load.
In order to solve the above problem, a matching circuit such as a trap circuit or a switch circuit is provided at the connection point 24 of the signal sources so as to obtain a matching in each frequency band. In this way, a signal output from one signal source may be prevented from being transmitted into the other signal source, thereby preventing an undesired decrease in the level of a signal being fed into an amplifying element. However, with the use of such a method, there have been found three problems which may be concluded as follows. A first problem is that it will be difficult to input a signal into an amplifying element without decreasing the level of the respective two signals to be inputted into the amplifying element. A second problem is that extra electronic parts in accordance with the addition of a matching circuit are required, hence it is difficult to reduce the number of the components in the oscillator and miniaturize the oscillator. A third problem is that the designing of the matching circuit becomes a time-consuming work.
It is an object of the present invention to provide an input switching amplifier and a frequency switching oscillator using the input switching amplifier, in which two signals having mutually different frequencies is inputted into an amplifying element without reducing the levels of the signals, thereby making it sure to produce an output having a high signal level.
It is another object of the present invention to provide an input switching amplifier and a frequency switching oscillator using the input switching amplifier, which does not require a matching circuit such as a trap circuit or a switch circuit at the connection point of two signal sources, thereby making it possible to reduce the number of needed electronic components, so as to produce an oscillator which is compact in size, and thus allowing the designing of necessary circuits in only a shortened time.
In order to achieve the above object, an input change-over type amplifier according to the present invention, comprises: an amplifying element having a first input terminal, a second input terminal, and one output terminal; a first grounding circuit which is connected to the second input terminal and is capable of being grounded at a high frequency with respect to the frequency of a signal being inputted into the first input terminal; a second grounding circuit which is connected to the first input terminal and is capable of being grounded at a high frequency with respect to the frequency of a signal being inputted into the second input terminal.
Further, in the input change-over type amplifier according to the present invention, the first or the second grounding circuit includes a capacitor.
Moreover, in the input change-over type amplifier according to the present invention, a self resonant frequency of the capacitor substantially corresponds with the frequency of a signal being inputted into the first or the second input terminal.
In addition, in the input change-over type amplifier according to the present invention, the first or the second grounding circuit includes a switch member.
Further, a frequency change-over type oscillator according to the present invention, comprises the above described input change-over type amplifier; a first signal source connected with the first input terminal; a second signal source connected with the second input terminal.
With the above constitution of the present invention, i.e., with the input change-over type amplifier and the frequency change-over type oscillator using the same, since two signals having different frequencies are allowed to be inputted into the amplifier via different paths, the two signals can be inputted into the amplifier without decreasing their signal levels, thereby making it possible to obtain an output of a high signal level.
Further, with the above constitution of the present invention, it is not necessary to provide a matching circuit at a connection point of signal paths. Therefore the number of electronic components can be reduced, and it is allowed to produce an oscillator which is compact in size and design a necessary circuit with only a shortened time.